Many times, hardwood flooring consumers and retailers have questions about how hardwood floors react to moisture in various regions of the country. Hardwood floors will expand and contract to different degrees according to varying levels of moisture in the environment and the wood itself. Hardwood flooring installed in regions with arid climates will react differently than hardwood floors in regions subject to seasonal high humidity levels. But that’s only part of the story. Here is a quick and simplistic primer on how hardwood floors are effected by moisture and the different variables that determine the degree of the effect.

Wood veneers are either plain sawn or rotary cut. The expansion or shrinkage of the wood varies:
a) According to the species.
b) According to where the cut comes from in the log, knowing that the heart wood is harder than the outer part where the sap wood is found.
c) According to the cutting season. Winter wood is generally harder than spring/summer wood.
d) According to the cut.
– Plain Sawn: The log is sliced form top to bottom or left to right. This generally used method is more economical and provides the best yield, but since the cut is across the rings, these veneers will expand easily.
– Quarter sawn: The log is divided in quarter and the cut is made across the grain. There is more waste with this method but the stability is better than with the plain sawn method.
– Rotary cut: The log is soaked in water then peeled, to make an analogy, like unrolling a roll of toilet paper. Because of this treatment the wood fibers have been weakened and are more prone to cracking. It is not recommended to use thick veneers that are rotary cut.
In addition, there are 2 important factors which are critical to insure the performance of a wood floor:

Moisture content. We specify a moisture content of 6% to 8% for the wood floors we sell. While the moisture content in Georgia is 10% to 12%, in most Western States (to the exception of the coastal area), the MC is between 4% to 6%. Because of these variations the wood is experiencing a lot of stress and acclimation times are critical. Although we make recommendations about the acclimation period, it is the responsibility of the installer to wait until the wood has reached an adequate MC content (within 2%) before proceeding with the installation.

Relative Humidity. For wood floors to behave normally, the Relative Humidity in the environment must be maintained between 30% and 60%. This is why, generally during the heating season, people might experience excessive shrinkage (solid wood) or surface checking and delaminating (engineered).

Most moisture related claims, generally from the Western States, are related to the lack of relative humidity also called hygrometry level. Most people are not knowledgeable about these issues and even professional flooring retailers often fail, often due to a lack of knowledge, to warn their customers about the limitations of wood flooring.
In spite of being more stable than solid floors, engineered floors will experience great stress under extreme lack of moisture. The solution is of course to control the environment and the best tool is humidifier.